1.field of the Invention PA1 This invention relates to production of delayed petroleum coke and more particularly to a reliable method of providing a feedstock which will produce a premium grade coke having a very low coefficient of thermal expansion. PA1 2. Description of the Prior Art PA1 The delayed coking of petroleum residual oils is well established in the industry, providing the maximum return from residual oils in that it yields both coke and more desirable liquid and gaseous products, such as liquefied petroleum gas, gasoline, and gas oil. Delayed coking has become even more important in recent years in that it has also been found to be an excellent route to the production of premium grade or needle coke, useful in the production of large graphite electrodes, from certain selected feedstocks which are generally high in aromatic content.
There are a number of factors that determine the quality of coke. For example, sulfur content, hardness, metals content, electrode electrical resistivity, and coefficient of thermal expansion are all factors in determining the quality and value of delayed coke. Although each of these factors is important, the coefficient of thermal expansion (CTE) is the primary factor in determining the value of coke. The lower the coke's CTE, the more valuable the coke, and the better for production of large graphite electrodes.
Premium coke has customarily been produced in delayed cokers from thermal tars. These tars are made by the thermal cracking of virgin, thermal cracked, and catalytically cracked gas oils. Attempts to make premium coke from gas oil without first thermally cracking the gas oil have generally been unsuccessful, and attempts to predict coke quality from feedstock properties have been unsuccessful for the most part. The lack of success in producing premium coke without thermally cracking the feedstock, combined with the inability to accurately identify and quantify components in coker feedstocks, has led the industry to the belief that a thermal cracking operation is needed in conjunction with a coker installation in order to produce premium coke. Recent trends in the petroleum refining industry, such as the increased use of fluid catalytic cracking units in place of thermal cracking units, have forced the industry to search for a feedstock for delayed coking that does not require a thermal cracking step prior to coking.
As previously mentioned, coker feedstocks normally comprise residual oil which has been subjected to various processing steps prior to introduction to the coker. The nature of these feedstocks is such that it is virtually impossible to analyze them, and because of their source, they are subject to variation even when they have been subjected to similar processing prior to coking. U.S. Pat. No. 3,759,822 describes a method for producing premium coke comprising coking a blend of a thermally or catalytically cracked heavy oil having a high aromatic content with a quantity of a pyrolysis tar under rather conventional coking conditions. U.S. Pat. No. 2,922,755 to Hackley describes a process for producing premium coke in which the feedstock is a blend of a highly aromatic thermal tar with one or more refinery residues, such as reduced crude or hydroformer bottoms. The coking processes described in these patents, as well as other variations of the basic coking process, have been utilized previously with varying degrees of success in the production of premium delayed petroleum coke. However, there has been a continuing need for a better understanding of the relationship between coker feedstock and product quality. In many cases, for unexplained reasons, product quality has failed to meet specifications even though the feedstock was from the same origin as earlier feedstocks which produced high quality product. In this regard, the primary measure of product quality, as was mentioned previously, is the linear coefficient of thermal expansion, or CTE. The value of this measurement, in order for the product to be designated a premium coke, is not precise, but it is generally considered that a CTE of less than about 5.0 .times. 10.sup.-7 per .degree. C is sufficient to designate the product as premium coke. However, the lower the CTE, the better, and in some cases, a batch of product having a particularly low CTE may be useful in blending product to produce an overall CTE of 5.0 .times. 10.sup.-7 or whatever the designated specification might be.
Prior to this invention, there has been no reliable test to determine feedstock quality, and the upstream processing of the feedstock has been the primary basis for selection. This method is not always effective, for a number of reasons, in producing a premium grade coke.
Thus, it is apparent that there has been a continuing need for a method of predicting and improving the quality of delayed coke prior to actually producing the coke, such as by determining a characteristic of the feedstock which correlates with product quality, and adjusting the feedstock if necessary to produce the desired characteristic in the feedstock prior to conducting the coking operation.